1. Field of the Invention
This invention relates to a multiquantum barrier field effect transistor (FET) having improved and distinguished characteristics.
2. Prior Art
High-electron mobility transistors (HEMTs) are attracting large interest for their excellent functional features such as high operating speed and low power consumption.
An HEMT device having a channel layer of Ga.sub.x In.sub.1-x As is expected to have a promising future because of its enhanced mobility in a low electric field, large saturation velocity, large band gap in a heterointerface and other outstanding characteristics in terms of current gain and cut-off frequency. Some of the papers reporting on this type of device include the following:
Paper No. 1: U. Mishra et al., IEEE, EDL, 9.41 (1988) PA1 Paper No. 2: J. Kuang et al., Appl. Phys. Lett. 57, 1785 (1990) PA1 Paper No. 3: Col. of Papers for 1992 spr., Ins. of Electronic Information and Telecommunication, 29p-ZP-6 (1992) PA1 Paper No. 4: K. Iga et al., Electron. Lett. 22, 1008 (1986)
3. Problems to be Solved by the Invention
An HEMT device having a channel layer of Ga.sub.x In.sub.1-x As is, however, accompanied by certain problems to be solved as described below.
Firstly, the relationship between the transfer conductance gm and the gate-source voltage Vgs of such a device exhibits a saturation at a certain Vgs(V.sub.s) and the value of gm tends to decrease in Vgs region larger than Vgs(V.sub.s).
Secondly, a device under consideration has a low-drain breakdown voltage due to large gate-leakage current. Incidentally, an AlGaAs/GaAs type device has a drain breakdown voltage of 7V, whereas an InGaAs/AlInAs type device shows a drain breakdown voltage of 4 to 6V.
On the other hand, Paper No. 3 as quoted below reports that the gm-Vgs relationship of a GaAs/InGaAs/AlGaAs type device can be improved by increasing the conduction band edge discontinuity .DELTA.Ec between AlGaAs and InGaAs increasing Al content of AlGaAs layer.
In InGaAs/AlInAs HEMT device, as is clearly shown in Paper No. 2 listed above and in FIG. 3 of the accompanying drawings of this specification, a large gm has been achieved and the breakdown has been improved by arranging its electron supplying layer as a modulation doped structure with a nondoped AlInAs layer and increasing the barrier height of the electron supplying layer. However, the tendency of easy saturation of gm and the tendency of lowering of gm in a large gate-source voltage can not be improved by such an arrangement.
There has been reported in Paper No. 5 as listed below that gm and other characteristics of a device of the type under consideration can be improved by using a GaInAs strained channel layer, lowering the conduction band edge of the channel and raising the heterobarrier height, i.e., the conduction band edge discontinuity .DELTA.Ec.
All these reports reveal a fact that the intrinsic performances of the device of type under consideration can be improved by (1) increasing the heterobarrier height and realizing sufficient confinement of two-dimensional electron gas into the channel layer, (2) increasing the gate Shottky barrier height and reducing the gate-leakage current. However, the conventional technique as described in Paper No. 2 quoted above does not provide any satisfactory improvement because gm tends to be easily saturated.
It is, therefore, an object of the present invention to provide an HEMT which is free from the material restrictions such as the electron affinities and other constants specific to the materials which take part in determining the barrier height of the electron supplying layer against the channel layer and the barrier height of the gate Schottky contact. By a sufficiently large barrier height, carriers can be satisfactorily confined to the channel layer and the gate-leakage current can be well suppressed so that the device can show excellent characteristics in terms of transfer conductance, breakdown voltage and other characteristics.